Isotopic constraints on the age and formation of a Palaeoproterozoic volcanic arc complex in the Kedougou Inlier, eastern Senegal, West Africa

The Palaeoproterozoic Kedougou basement in the Mako area in eastern Senegal consists of four major rock complexes: 1) high-grade amphibolitic and gneissic rocks occuring as large xenoliths (the Sandikounda amphibolite-gneiss complex); 2) voluminous bimodal volcanic rocks (the Mako volcanic-plutonic complex); 3) the Sandikounda layered plutonic complex; and 4) trondhjemitic to calc-alkaline intrusive granitoids (the Laminia-Kaourou plutonic complex). Zircons from xenoliths of the Sandikounda amphibolite-gneiss complex yielded ages of similar to 2200 Ma, which are slightly older than those of the Mako Supergroup volcanic rocks. These xenoliths also have epsilon Nd(t) values ranging from +2.4 to +4.1. The volcanic sequence represents an important bimodal volcanic event with Mg-tholeiites and subordinate felsic calc-alkaline lavas having epsilon Nd(t) values ranging from +2.6 to +3.8. The plutonic intrusions yield ages between 2158 and 2079 Ma with positive epsilon Nd(t) values of +2.6 to +4.7. Isotopic evidence indicates that the Mako Palaeoproterozoic crust is a juvenile, are-related terrain derived from a depleted mantle source, emplaced between 2300 and 2079 Ma and subsequently accreted to the growing West African Craton. An early magmatic event, represented by the protolith of the amphibolite and the gneiss, is inferred to represent the initial stage of this Palaeoproterozoic crustal growth and may represent the root of the Mako volcanic-plutonic complex. Thus, the Birimian-Eburnian development of the western part of the Kedougou Inlier is constrained to the interval 2300-2079 Ma; this is slightly older than other parts of the Birimian-Eburnian domains elsewhere in western Africa. (C) 1997 Elsevier Science Limited.